This application is based on Japanese Patent Application No. 2000-294711 filed on Sep. 27, 2000, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates in general to a vibration damper and a method of producing the same, and more particularly to improvements of weather resistance and other properties of a vibration damper of composite type including a vulcanized-rubber body and a polyamide-resin member which are integrally bonded together, and a method suitable for producing such an improved composite type vibration damper.
2. Discussion of Related Art
Various types of vibration dampers have been employed on automotive vehicles, railcars and other vehicles, for the purpose of preventing or reducing vibrations and shocks transmitted to rigid components of the vehicles. There have been researches, developments and applications of a variety of such vibration dampers of metal-rubber composite type, each including a mounting member made of a metallic material and an elastic body formed of a rubber material which is bonded to the mounting member in the process of vulcanization of the rubber material. Recently, however, there have been employed various vibration dampers of resin-rubber composite type each including a mounting member made of a resin material in place of a metallic material, particularly for automotive vehicles, for the purpose of reducing the weight and cost of manufacture of the vibration damper.
In the vibration dampers of the resin-rubber composite type described above, the mounting member is formed of a resin material selected from among resin materials that exhibit particularly high degrees of heat resistance and durability. Presently, the use of a polyamide-resin for the mounting member is considered advantageous for an improved effect of reinforcement of the mounting member by a glass fiber, increased degrees of injection-molding formability and chemical resistance of the mounting member, and a reduced cost of manufacture of the mounting member.
However, the polyamide resin has a problem that its mechanical properties tend to be deteriorated due to water absorption under a wet and humid atmosphere, and upon exposure to calcium chloride used as a snow-melting agent or an anti-freezing agent sprayed on road surfaces in the winter season. The polyamide resin has further problems, such as deterioration due to oxidization upon exposure to heat, and reduction of mechanical strength upon exposure to ultraviolet rays.
In view of the above-indicated problems of the polyamide resin when used for the mounting member in the vibration damper of the resin-rubber composite type, it is generally considered effective to add a suitable filler such as a carbon black to the polyamide resin, for improving the weather resistance of the vibration damper. However, the use of such a filler does not permit satisfactory improvements of the properties of the mounting member such as water resistance and resistance to a snow-melting agent (calcium chloride). Accordingly, the application of the vibration damper including the mounting member made of a polyamide resin and the elastic body made of a rubber material is considerably limited. Namely, the vibration damper of this resin-rubber composite type can be used only in an environment wherein the mounting member is comparatively less likely to be exposed to a wet and humid atmosphere and calcium chloride.
The present inventors have conducted intensive study and research in an effort to solve the problems of the conventional vibration damper of resin-rubber composite type including a vulcanized-rubber body and a polyamide-resin member which are integrally bonded together. As a result of the study and research, the inventors found it possible to significantly improve the water resistance, weather resistance and resistance to snow-melting agents of the polyamide-resin member, by covering an exposed surface of at least the polyamide-resin member with a layer of an elastic material.
The present invention was made on the basis of the finding indicated above. It is therefore a first object of the present invention to provide a vibration damper of composite type which includes a vulcanized-rubber body and a polyamide-resin member that are integrally bonded together, and which is improved in its properties such as water resistance, weather resistance and resistance to snow-melting agents. It is a second object of the present invention to provide a method of producing a vibration damper of composite type which includes a vulcanized-rubber body and a polyamide-resin member that are integrally bonded together, and which is improved in its properties such as water resistance, weather resistance and resistance to snow-melting agents.
The first object indicated above may be achieved according to a first aspect of the present invention, which provides a vibration damper of composite type including a vulcanized-rubber body formed of a vulcanized rubber material and a polyamide-resin member formed of a polyamide resin material. The vulcanized-rubber body and the polyamide-resin member are integrally bonded together. A coating layer is formed of an elastic material with a predetermined thickness, so as to cover an exposed surface of at least the polyamide-resin member.
The vibration damper of composite type constructed according to the present invention as described above is characterized in that the coating layer formed of a suitable elastic material with a suitable thickness covers the exposed surface of the vibration damper which includes at least the exposed surface of the polyamide-resin member, which may serve as a mounting member at which the vibration damper is mounted. Accordingly, the polyamide-resin member is effectively protected against a direct exposure or contact thereof to or with raindrops, water moisture contained in the atmosphere, or calcium chloride used as a snow-melting agent or an anti-freezing agent, so that the polyamide-resin member is kept free from a problem of deterioration of its dynamic properties which would take place due to the exposure to such aqueous components (e.g. rain drops, water, moisture, snow-melting agent, ect.). In addition, the coating layer is effective to prevent deterioration of the polyamide-resin member due to oxidization under heat, and undesirable reduction of the mechanical strength due to exposure to ultraviolet rays.
Thus, the vibration damper according to the present invention can be used for a long time even in the presence of aqueous components, such as, for example snow-melting agents (e.g., calcium chloride) while maintaining a sufficiently high degree of overall mechanical strength, owing to the coating layer which is significantly effective to protect the polyamide-resin member, serving as the mounting member of the vibration damper, against deterioration of the mechanical properties.
In the present vibration damper, the coating layer is formed of an elastic material, so that even where the exposed inner surface of the vulcanized-rubber body, as well as the exposed outer surface of the polyamide-resin member, is coated by the coating layer, the coating layer will not disturb the intended vibration damping action of the vulcanized-rubber body and the vibration damping characteristics of the vibration damper. That is, the coating layer formed of the elastic material is capable of elastically deforming following the elastic deformation of the vulcanized-rubber body upon application of a vibrational load to the vibration dampen. Further, the elastic nature of the coating layer effectively prevents the cracking or separation of the coating layer from the polyamide-resin or vulcanized-rubber body, thereby making it possible to maintain high degrees of water resistance, weather resistance and resistance to the snow-melting agent of the polyamide-resin member.
Further, the coating layer covering the exposed surface of at least the polyamide-resin member effectively improves the impact or shock resistance and the chipping resistance of the polyamide-resin member.
According to one preferred form of the vibration damper of the present invention, the elastic material of the coating layer is a rubber material or a thermoplastic elastomer. In this form of the invention, the weather resistance and heat resistance (i.e., resistance to aging due to heat) of the coating layer per se can be advantageously improved by adding a carbon black and/or an anti-aging agent to the rubber material or thermoplastic elastomer of which the coating layer is formed on at least the polyamide-resin.
According to another preferred form of the present vibration damper, the predetermined thickness of the coating layer is selected within a range of 10-3000 xcexcm. If the thickness of the coating layer is smaller than 10 xcexcm, the coating layer does not permit an appreciable improvement of the water resistance, weather resistance and resistance to the snow-melting agent of the polyamide-resin member, and it would be considerable difficult to form the coating layer with uniform thickness on the polyamide-resin member. If the thickness of the coating layer is larger than 3000 xcexcm, the coating layer would disturb the vibration damping action of the vibration damper, resulting in a failure of the vibration damper to exhibit the intended vibration damping characteristics.
According to a further preferred form of the vibration damper of the present invention, the elastic material of the coating layer is selected from among rubber materials and thermoplastic elastomers which have a low water-absorption property. In this instance, the deterioration of the mechanical properties of the vibration damper due to water absorption of the polyamide-resin member can be effectively prevented.
According to a yet further preferred form of the present vibration damper, the elastic material is selected from among rubber material and thermoplastic elastomers which have a low oxygen permeability. In this case, the heat resistance and durability of the polyamide-resin member can be effectively improved.
The second object indicated above may be achieved according to a second aspect of this invention, which provides a method of producing a vibration damper of composite type as described above, the method including the steps of: preparing an integral assembly consisting of the vulcanized-rubber body and the polyamide-resin member which are integrally bonded together; applying a solution of an unvulcanized rubber material to the exposed surface of at least the polyamide-resin member, to form a layer of the unvulcanized rubber material on the exposed surface; and vulcanizing the layer of the unvulcanized rubber material to form the coating layer on the exposed surface of at least the polyamide-resin member. The present method facilitates the formation of the coating layer of the rubber material so as to cover the exposed surface of at least the polyamide-resin member, which effectively improves the water resistance, weather resistance and resistance to calcium chloride of the polyamide-resin member.
The second object may also be achieved according to a third aspect of the present invention, which provides a method of producing a vibration damper of composite type (as defined above), including the steps of: preparing an integral assembly consisting of the vulcanized-rubber body and the polyamide-resin member which are integrally bonded together; applying a solution of a thermoplastic elastomer to the exposed surface of at least the polyamide-resin member; and subjecting the solution of the thermoplastic elastomer to a drying operation, to form the coating layer on the exposed surface of at least the polyamide-resin member. The present method facilitates the formation of the coating layer of the thermoplastic elastomer so as to cover the exposed surface of at least the polyamide-resin member.